Lithium ion secondary cells have secured their places as high-capacity and light-weight power sources essential for mobile devices, electric vehicles, and so on. However, current lithium ion secondary cells employ as their electrolytes, mainly, combustible organic electrolytic solutions and, therefore, raise concerns about the risk of ignition or the like. As a solution to this problem, developments of lithium ion all-solid-state cells using a solid electrolyte instead of an organic electrolytic solution have been promoted (see, for example, Patent Literature 1).
However, an issue of concern with lithium is global price increase of raw materials thereof. To cope with this, sodium has attracted attention as a material to replace lithium and there is proposed a sodium ion all-solid-state cell in which a NASICON-type sodium ion-conductive crystal made of Na3Zr2Si2PO12 is used as a solid electrolyte (see, for example, Patent Literature 2). Alternatively, beta-alumina-based solid electrolytes, including β-alumina and β″-alumina, are also known to exhibit high sodium ion conductivity. These solid electrolytes are also used as solid electrolytes for sodium-sulfur cells.
From the viewpoint of increasing the energy density per unit volume of the sodium ion all-solid-state cell or the like, it is desirable that such a solid electrolyte as above be formed into a sheet.